Honeycomb structures are widely used as internal combustion engine exhaust gas purification catalyst carriers or deodorizing catalyst carriers for vehicle exhaust gas and the like. Conventionally, with such honeycomb structures, in the case of using as a vehicle exhaust gas purification catalyst carrier for example, the walls partitioning the cell passages have conventionally been generally formed in flat shapes to reduce pressure loss.
However, in recent years, in accordance with stricter emission standards based on environmental issues, purification capabilities of exhaust gas purification catalysts for vehicles have come to be viewed as being more important than pressure loss properties. Accordingly, development of engines to reduce the amount of emission of harmful substances such as hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx), and so forth, and improvement of three way catalysts have progressed, and emission of harmful substances is on a decline due to the effects of both of these.
Thus, while the overall emissions during operation of the engine are decreasing, the amount of emission of harmful substances immediately after starting the engine is being examined more closely. With the FTP-75 cycle which is the restriction driving cycle in the USA, for example, 60% to 80% of the total amount of exhaust is discharged within the Cold-Transient mode which is the first 140 seconds after starting the engine.
One cause is that, immediately following starting the engine in particular, the exhaust gas temperature is low and the catalyst is not activated sufficiently, so the harmful substances pass through the catalyst without being purified. Also, another factor is that the fuel combustion state is not stable in engines immediately after starting, and the A/F (air/fuel ratio) of the exhaust gas, i.e., the ratio of oxygen in the exhaust gas, which is an important factor that affects the purification capabilities of the three way catalyst, changes.
Accordingly, various attempts are being made to speedily raise the temperature of the catalyst immediately after starting the engine, such as placing the catalyst where the temperature of the exhaust gas is high by positioning the catalyst as close to the engine as possible, making the cell partitions thin to lower the thermal capacity of the catalyst carrier itself, increasing the cell concentration of the carrying member in order to increase the amount of area of contact between the catalyst and exhaust gas while speedily absorbing the heat of the exhaust gas, and so forth.
However, with conventional normal honeycomb structures, the walls are almost always formed with a flat shape so that the cell passages are straight tubes, in order to reduce pressure loss. Accordingly, measures are taken to increase the contact area between the walls and the exhaust gas by making the walls thinner and increasing the number of cells, but improvement of purification capabilities has been limited, such as there being limitations to the increase in contact area, sufficient purification effects for harmful substances in the exhaust gas has not been observed, and so forth.
Further, in the event that the amount of catalyst being carried is increased to improve the purification capabilities, not only are great amounts of platinums which are the catalyst component used, which leads to increased costs, but also the catalyst layer becomes thicker, which means that the percentage of catalyst which can actually come into sufficient contact with the exhaust gas decreases, so the increase in purification capabilities hoped for cannot be obtained. Also, while the initial capabilities of the catalyst are high, there is an even higher concentration of precious metals, and the catalysts tend to coagulate one with another over time, leading to the problem of short usage life span.
Accordingly, JP-A-58-43238 discloses a ceramic honeycomb structure wherein the cell passages have been made in a meandering form from the entrance to the exit, so as to increase interaction between the cell passage walls and the fluid flowing therethrough and thus improve purification capabilities by increasing the surface area within the cell passages, and a method for manufacturing a honeycomb structure wherein the cell passages have been made in a meandering form in the cell passage direction (referring to the passage direction of the cell passages) by causing extrusion molding members to perform rotational vibration within the cell passage cross-sections.
However, with the honeycomb structure disclosed in JP-A-58-43238, the walls partitioning the cell passages one from another have a curved form, but no protrusions or recessions have been provided to the surface of the walls. Accordingly, the increase in the surface area within the cell passage is limited to the length of the cell passage lengthened by making the cell passage to meander, so marked improves in catalyst capabilities are not expected.
Also, JP-A-3-151049 discloses a ceramic honeycomb structure wherein the walls of the periphery portion of the honeycomb structure are flat, and only the walls at the center portion are raised and lowered, thereby increasing the interaction between the exhaust gas and the walls so as to increase purification effectiveness, and also forming the periphery portion walls thicker so as to increase the strength against external pressure and the holding strength.
However, with the honeycomb structure disclosed in this JP-A-3-151049, while the cell passages themselves are raised and lowered in the cell passage direction, the walls are not formed so as to be raised and lowered in the cross-sectional direction of the cell passages. Accordingly, as with the case of the honeycomb structure disclosed in JP-A-58-43238, the increase in surface area within the cell passage is restricted to the length by which the cell passage length has been increased by forming protrusions and recessions in the cell passage direction, so marked increase in catalyst capabilities are not expected.
Further, JP-A-5-123580 discloses a honeycomb structure wherein the walls at the center portion are formed in an undulated shape in both directions of the cell passage direction and the cross-sectional direction perpendicular to the passage direction, and also wherein the recessions and protrusions of the undulated wall are synchronized so as to face in the same direction in the direction of the cell passage.
With the honeycomb structure according to JP-A-5-123580, in addition to increased surface area by essential extending of the cell passage length as with JP-A-58-43238 and JP-A-3-151049, the walls are undulated in the direction perpendicular to the cell passage direction as well, thereby increasing surface area.
However, the recessions and protrusions thereof are synchronized in the direction of the cell passage, so the shape of the cross-section at arbitrary positions along the cell passage is the same. Accordingly, the flow of a fluid within the cell passage readily becomes a stationary flow, and consequently there is a problem that it becomes difficult to aggressively increase the interaction between the fluid flowing through the cell passages and the wall faces.
Also, JP-A-52-119611 discloses deforming walls for the purpose of adjusting thermal stress or deformation due to mechanical stress on the plane perpendicular to the longitudinal direction of the cells (passage direction), but due to the same problem as with that in JP-A-5-123580, does not contribute to improved catalyst capabilities. Also, the description in JP-A-52-119611 that the amplitude of the wall deformation (sine wave) deformations is smaller than the wall thickness reduces stress focusing on the deformed portions of the walls, but does not agree with the aggressive increase of interaction between exhaust gas and the walls, with is the essence of the present invention.
Further, with metal honeycomb structures wherein the cell formations of stainless heat-resistant steel are undulated forms, structures are being proposed wherein a great number of small round indentations are formed in the cell passage direction with a certain spacing therebetween, in the direction orthogonal to the cell passage direction. However, with this structure, great turbulence effects cannot be expected, and also even in the event that catalyst is carried, the grooves are small, so the catalyst layer fills in the grooves, thereby reducing the effects of forming the grooves. There are hardly any pores in the case of metal, so coating with a catalyst layer of γ alumina results in a coat with frequent collecting in corner portions like grooves.
Now, honeycomb structures which are small, light, and use less amount of catalyst while manifesting excellent properties of transfer effectiveness and so forth are also being desired, in addition to objects other than the above-described exhaust gas purification, such as for honeycomb structures used as chemical reaction catalyst carriers for gas modifying or the like wherein pressure loss is not a great impedance in usage.
The present invention has been made in light of the above-described problems of the conventional art, and it is an object thereof: to provide a honeycomb structure wherein the surface area of the walls is increased while making the flow of fluid within the cell passages complex so as to increase the interaction between the fluid and walls, which further has mechanical strength and excellent heat and shock resisting properties sufficient to allow placement near an engine to serve for purifying exhaust gasses of engines and the like; and to provide the manufacturing method thereof.